1. Field of the Invention
This invention relates to electrochromic display devices which are suitable for displaying figures or letters or also for X-Y matrix display and more particularly, to green color electrochromic display devices.
2. Description of the Prior Art
Electrochromic displays are of the non-luminous type and are able to display through reflected or transmitted light, so that they are advantageous in that one feels less fatigued on observation of the display over a long term, coupled with another advantage that the driving voltage is relatively low with a small power consumption.
Liquid electrochromic displays are known inculding a display in which viologen is used as an active substance and potassium ferrocyanide is used as an auxiliary redox material. In this display, the electron transfer reaction proceeds reversibly within the two pairs of redox systems consisting of viologen/potassium ferrocyanide, thereby forming or erasing a color.
In this type of electrochromic display, there is known a fairly long-lived system without involving "erasure failure" with regard to reddish purple electrochromic displays. For the display of variable messages or data, there are needed a variety of colors including green and blue colors.
Green color electrochromic displays are known. In one such display, there is used p-cyanophenyl viologen (which may be sometimes abbreviated as p-CV hereinafter) as an active substance, which exhibits a green color in the reduced state in an aqueous solution. For instance, Japanese Laid-open Patent Application No. 47-13293 teaches use of p-cyanophenyl viologen dichloride (p-CV2Cl).
The above type of liquid electrochromic display is so designed that a pair of facing electrodes, at least one of which is a display electrode, are in contact with an electrochromic solution. The color formation and erasure reaction of p-cyanophenyl viologen proceeds according to the oxidation and reduction reaction of the following formula (1) ##STR1##
The reduced material formed on the electrode according to the formula (1) has a green color. In order to solve the problem of the "erasure failure" on the electrode, there is added an alkali ferrocyanide, as an auxiliary redox compound, which acts to facilitate the color erasure. The mechanism of facilitating the color erasure is considered to proceed according to the following formula (2): ##STR2##
In practice, when the concentration of p-CV is increased in the green color electrochromic display making use of two pairs of redox systems of P-CV and an alkali ferrocyanide such as, for example, sodium ferrocyanide, the solution itself assumes the color. If the concentration further increases, flaky green crystals appear. This will lower the contrast and cause the display to be unfit for use. Accordingly, the molar concentration of p-CV must be below about 0.001 mole, which is disadvantageous in that a dense green color display cannot be obtained (Japanese Laid-open Patent Application No. 47-13293). In Table 1, there is shown a color change of solutions comprising varying concentrations of p-CV2Cl and sodium ferrocyanide, Na.sub.4 [Fe(CN).sub.6 ].
TABLE 1 ______________________________________ p-CV2Cl 0.001 0.002 Na.sub.4 [Fe(CN).sub.6 ] M M 0.004 M 0.005 M 0.010 M ______________________________________ 0.1 M light light light blue separ- separ- yel- blue (after ating of ating of low stand- green green ing for 2 crystals crystals days, the sol- ution became darkened 0.01 separ- separating ating of of green green crystals crystals ______________________________________
As will be seen from Table 1, the elctrochromic display using P-CV/sodium ferrocyanide has the drawback that the solution assumes the color when p-CV is used at a concentration of 0.001M and separates out green crystals irrespective of the concentration of the alkali ferrocyanide when p-CV is used at a concentration of 0.005M, i.e. green crystals in the form of flakes appear.
Thus, the known green electrochromic displays which make use of two pairs of redox systems consisting of p-CV/alkali ferrocyanide, undesirably involve the coloration of the solution or the formation of green crystals, so that the molar concentration of p-CV cannot be increased to a satisfactory extent with an insufficient density of the green color in the display.